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Background studies at ATF2 and the BDS/IR -focus on neutrons-

Background studies at ATF2 and the BDS/IR -focus on neutrons-. Hayg GULER, Marc VERDERI LLR - Ecole polytechnique / IN2P3 With ANR funding through ANR-06-BLAN-0027 IWLC 2010. Introduction [1/2]. Background studies for ILC/CLIC are carried on with simulation tools eg : Geant4, FLUKA, …

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Background studies at ATF2 and the BDS/IR -focus on neutrons-

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  1. Background studies at ATF2 and the BDS/IR-focus on neutrons- Hayg GULER, Marc VERDERI LLR - Ecolepolytechnique / IN2P3 With ANR funding through ANR-06-BLAN-0027 IWLC 2010

  2. Introduction [1/2] • Background studies for ILC/CLIC are carried on with simulation tools • eg : Geant4, FLUKA, … • And detailed studies have already been conducted • See for example Adrian Vogel’s thesis (with Geant4) • Reliability of simulation is however questioned • Do MC and data differ by large factors ? • What is the case for the broadly used Geant4 software ? Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  3. Example of background simulation Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  4. Introduction [2/2] • Activity at ATF2 to address some of the reliability questions of Geant4 • The approach is to consider a real case situation • And “simply” measure, simulate, and compare • What can be learned at ATF2, where e- beam energy is 1.3 GeV ? • At CLIC/ILC the maximum energy is O(TeV) • But in dense materials, this energy rapidly degrades to low energy EM particles • With high multiplicity • Neutron production through photo-nuclear effects is then dominated by this low energy regime • In the beam dump area • But also in dense materials close to the IP • ATF2 can say something about this neutron production regime • An other aspect is to learn and exercise in a real case methods and techniques needed for background simulation • As straightforward simulations are inefficient in getting workable statistics • This is needed for neutron background simulation • But (even if not presented today) should be useful for EM background modeling, in trying to correlate background level with beam parameters Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  5. Experimental setup and measurement apparatus Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  6. Experimental setup and apparatus • Measure neutrons exiting from ATF2 beam-dump • Beam dump made of joint slabs iron made [~(2m)3] • And has a small central part made of copper • Measurement devices: • Plastic (BC-408) and pure CsIscintillators (both provided by Saint Gobain), read by photomultipliers • CsI crystal equiped with filter to cut the slow light component • Fast, and different response to neutron • Subject to different systematic effects => allow for cross-check • Plastic : for fast neutrons, CsI : intermediate • Record the time-dependent signal • Ie signal waveform, sampling at 1 G-sample/s • Absolute amplitude and time dependent signal shape tell about neutron production and transport in beam dump Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  7. Measurementdevices • Modules list: • Installation example: Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  8. Examples of observed signals near dump(trigger = kicker) Slow neutrons from dump CsI Fast neutrons from dump plastic 500 ns Direct EM peak from line Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  9. Simulation setup and practice Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  10. Simulation setup • Geant4 version and physics: • Use Geant4, v9.3 – ref 06 • Physics list considered : • QGSP_BERT_HP, to start with • There are other ones a priori suitable • Implement detailed description of measurement modules, and realistic description of beam dump • Use “splitting” technique, also called “geometrical biasing” • To get workable statistics Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  11. Illustration of the « splitting » technique 50 incident e-, no splitting 50 incident e-, withsplitting Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  12. Illustration of the « splitting » technique 50 incident e-, no splitting 50 incident e-, withsplitting In this case, biased simulation is about 3 order of magnitudes more Efficient than the « straightforward » simulation. Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  13. Experimental effects and measurements Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  14. Experimental effects • Experimental effects can be large • And are still under work at present • Effects under study: • Calibration • Calibration done with cosmic rays • Attenuation by cables • Up to 50 meters long at present  • But fortunately rather small effect for slow signal like average of neutron signals • Scintillation saturation (Birks’ effect) in plastic scintillators • Large effect for neutrons ! • Fast neutrons detected as n → p, with high dE/dx for p Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  15. Size of Birks saturation effect • Birks saturation : dEvis/dx ~ dEdep/dx / (1+kBdEdep/dx) • Simulated waveform in plastic scintillator • Birks constant kB = 1.15∙10-2 g/cm2/MeV • value in BC-408 plastic, as measured in ChinesePhysics C (HEP & NP), 2010, 34(7) 988-992 • Very large effect ! • Size of systematiceffect on thisreduction to beestimated. Deposited energy Visible energy Birks Effect ! Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

  16. Preliminary Geant4/data comparison Plastic DATA Geant4 • Experimental setup: • Plastic, CsI, plastic • 70 cm to dump on lateral side (opposite to Shintake photon detector) • Plots normalized to 109 incident 1.3 GeV e-. • Significant differences CsI dump  70 cm P C P detectors e- beam Plastic • But Geant4 reproduces the gross features, and is not away by order of magnitudes.

  17. Conclusion • A study is ongoing at ATF2 to evaluate the physics reliability of Geant4 about neutron production and transport in a context of background modeling • Comparison between Geant4 (QGSP_BERT_HP physics list) and data are still preliminary at this stage • More work needed to have systematic effects on measurements under control • More Geant4 physics lists to be tested • Simulation and data differ • But gross-features are reproduced by the simulation • Simulation and data are not apart from order of magnitudes • They are typically within a factor 2 to 5 • This is, we hope, a startof the response for the MDI needs Marc Verderi - LLR, Ecole polytechnique / IN2P3 - IWLC 2010

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